I am running 12s, Castle ICE2 80HV in gov mode with 500kv motor (Scorpion) and stock 15t pinion. Using BeastX with MKS servos @ 6v through Castle pro BEC. Everything on the heli is stock with the exception of Edge 623 main blades and KBDD tail blades.

The heli spools up and hovers nicely. Transition to forward flight is good. Change head speed in ST1 and ST2 to approx 2000 to 2100 according to calculator (never actually tached) all good. The Heli will handle easy aerobatics, stall turns and Loops no problems.

Now comes the problem. From an inverted hover (still stable) I pull negative collective and push the heli away in a gentle rainbow. Just past the top of the arc, I center the collective and continue to push the cyclic to return to upright level hover. As soon as I add positive collective at the bottom of the rainbow, the heli begins to vibrate. I can hear it as soon as it starts and when I back the heli in towards myself, I can see the vibration in the tail fin and skids. The vibration will not stop in the air. I have to land and take back off to get it to stop. This is 100% repeatable, several people have flown the heli and it always happens.

I have been through the heli mechanically and nothing jumps out. No main shaft play. bearings feel good. I did add an extra torque tube bearing just to see if it made a difference. Drive gears and autorotation all feel good with no slop or play.

I am at a loss on this one. Has anybody experienced a problem like this? Suggestions?